Astronomers discover a new black hole in our galaxy

Oct 05, 2012 by Francis Reddy

(Phys.org)—NASA's Swift satellite recently detected a rising tide of high-energy X-rays from a source toward the center of our Milky Way galaxy. The outburst, produced by a rare X-ray nova, announced the presence of a previously unknown stellar-mass black hole.

"Bright X-ray novae are so rare that they're essentially once-a-mission events and this is the first one Swift has seen," said Neil Gehrels, the mission's principal investigator, at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is really something we've been waiting for."

An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months. The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole.

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The rapidly brightening source triggered Swift's Burst Alert Telescope twice on the morning of Sept. 16, and once again the next day.

Named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius. While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region.

Ground-based observatories detected infrared and radio emissions, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.

The nova peaked in hard X-rays—energies above 10,000 electron volts, or several thousand times that of visible light—on Sept. 18, when it reached an intensity equivalent to that of the famous Crab Nebula, a supernova remnant that serves as a calibration target for high-energy observatories and is considered one of the brightest sources beyond the solar system at these energies.

Swift J1745-26 with labels and scale of moon as it would appear in the field of view from Earth. Credit: NASA/Goddard Space Flight Center/S. Immler and H. Krimm

Even as it dimmed at higher energies, the nova brightened in the lower-energy, or softer, emissions detected by Swift's X-ray Telescope, a behavior typical of X-ray novae. By Wednesday, Swift J1745-26 was 30 times brighter in soft X-rays than when it was discovered and it continued to brighten.

"The pattern we're seeing is observed in X-ray novae where the central object is a black hole. Once the X-rays fade away, we hope to measure its mass and confirm its black hole status," said Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, who currently is working with other Swift team members at Penn State in University Park, Pa.

The black hole must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, sun-like star. A stream of gas flows from the normal star and enters into a storage disk around the black hole. In most LMXBs, the gas in the disk spirals inward, heats up as it heads toward the black hole, and produces a steady stream of X-rays.

But under certain conditions, stable flow within the disk depends on the rate of matter flowing into it from the companion star. At certain rates, the disk fails to maintain a steady internal flow and instead flips between two dramatically different conditions—a cooler, less ionized state where gas simply collects in the outer portion of the disk like water behind a dam, and a hotter, more ionized state that sends a tidal wave of gas surging toward the center.

"Each outburst clears out the inner disk, and with little or no matter falling toward the black hole, the system ceases to be a bright source of X-rays," said John Cannizzo, a Goddard astrophysicist. "Decades later, after enough gas has accumulated in the outer disk, it switches again to its hot state and sends a deluge of gas toward the black hole, resulting in a new X-ray outburst."

This phenomenon, called the thermal-viscous limit cycle, helps astronomers explain transient outbursts across a wide range of systems, from protoplanetary disks around young stars, to dwarf novae—where the central object is a white dwarf star—and even bright emission from supermassive black holes in the hearts of distant galaxies.

Swift, launched in November 2004, is managed by Goddard Space Flight Center. It is operated in collaboration with Penn State, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Va., with international collaborators in the United Kingdom and Italy and including contributions from Germany and Japan.

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User comments : 14

1, Why isn't the X-ray light heavily red shifted? Since gravitational red shift of objects near the event horizon should be shifted infinitely towards the red end of the spectrum as distance from the eH decreases.

2, why/how does a binary star reach the configuration shown in the video, seeing as how we all know the mass of the black hole is actually less than the parent star which formed the black hole. It becomes apparent that the black hole must have formed elsewhere, and was then "captured" by the host star it is now co-orbiting. Otherwise, if it's massive enough to strip off the outer layers of the sun-like star now, then it should have been massive enough to do that to begin with, when it was a regular star; in which case being a black hole has nothing to do with the classic artists' impression, since any nearby star with a high enough gravity field would do the same thing anyway.

Cantdrive85. Can I ask why "any claim of a black hole or neutron star is based on a long line of ill informed presumptions"? I am not asking to say you are wrong, I am asking because I am also "ill informed" and I do not wish to simply make presumptions because they fit what I think should be right. Or any one else care to answer this for me either in the affirmative or negative, I would greatly like to see the evidence for both arguments actually.

@Luker, it will be red-shifted as it moves away, and blue-shifted as it moves toward us. It couldn't be close enough to the central BH to show gravitational red-shifted without being completely disrupted.

Otherwise, if it's massive enough to strip off the outer layers of the sun-like star now, then it should have been massive enough to do that to begin with, when it was a regular star; in which case being a black hole has nothing to do with the classic artists' impression, since any nearby star with a high enough gravity field would do the same thing anyway.

This one is simple, and actually ties into the X-ray nova behavior. When the star that became a black hole was a main sequence star, outflowing gas and the light, would have prevented gas from the second star from reaching the brighter star. Once it became a black hole, with no inflowing gas, there would be no light and no wind. Gas from the second star now starts arriving.

This gas accumulates in a torus. Orbital mechanics eventually move the torus in close to the black hole, and it gets eaten. Cycle, rinse, repeat.

Why isn't the X-ray light heavily red shifted? Since gravitational red shift of objects near the event horizon should be shifted infinitely towards the red end of the spectrum as distance from the BH decreases.

Answer: Of course it is. If you read the article, the X-rays started at the high end of the spectrum. (Maybe an X-ray we see as 10 kV started out as 20 kV, no way to tell.) As the outburst ages, the energy of the X-rays we see drops. Maybe a 20 kV X-ray from deep in the black holes gravity well arrives with only 1 kV of energy.

There are no absorption lines at those frequencies/energies that can be used to determine what the original energy was.

I think the interesting thing is that the gas flowing into the b.h. and emitting the x-rays (which im sure is a lifetime of fascinating work on its own) is probably not hydrogen, if it we're i speculate black holes would be much easier to see due to relatively high levels of background protons anywhere near the vicinity of other matter. possibly the hydrogen is being energized enough as it enters the b.h. but only in large quantities. or we dont have xray detection technology on par with the level needed to see such minute xray concentrations.

There are no absorption lines at those frequencies/energies that can be used to determine what the original energy was.

Ah, my point exactly, for all you know, the frequency of the light when it is created could be some absurd number humans have never dreamed of, never mind observe.

The amount of Red Shift should increase exponentially as light sources get closer to the eH by a factor of one or two decimal places.

So light coming from just outside the eH should have originated with a wavelength equal to the Planck length, because I assume the boundary of relativity and quantum theory must meet at the eH of a black hole, that is agree with one another, else there would be no sense to the matter at all.

However,you couldn't necessarily use that as a tool to "weigh" the black hole or measure it's "real" event horizon, because the photons could have been absorbed and re-emitted by other materials in the accretion disk before taking the direct trip to us on Earth.

The amount of Red Shift should increase exponentially as light sources get closer to the eH by a factor of one or two decimal places.

So light coming from just outside the eH should have originated with a wavelength equal to the Planck length, ...

It depends on the size of the black hole. If it is large, the tidal force near the event horizon is small, it is sometimes noted that an astronaut could cross that of a SMBH without discomfort. The heating would only raise the temperature slightly as it approaches the event horizon so what we would see as the redshift increases is a thermal peak that first gets hotter but then cools as the increase in redshift exceeds the rate of heating.

When the star that became a black hole was a main sequence star, outflowing gas and the light, would have prevented gas from the second star from reaching the brighter star. Once it became a black hole, with no inflowing gas, there would be no light and no wind. Gas from the second star now starts arriving.

This gas accumulates in a torus. Orbital mechanics eventually move the torus in close to the black hole, and it gets eaten. Cycle, rinse, repeat.

Correct.

Gave it a 5.

I had already considered this, I just didn't write it in the prior post.

Yet it doesn't change the situation that there's nothing "magical" about black holes, at least within known physics and proper applications thereof.

It depends on the size of the black hole. If it is large, the tidal force near the event horizon is small, it is sometimes noted that an astronaut could cross that of a SMBH without discomfort. The heating would only raise the temperature slightly as it approaches the event horizon so what we would see as the redshift increases is a thermal peak that first gets hotter but then cools as the increase in redshift exceeds the rate of heating.

Gravitational acceleration at the Schwarzchild radius for the Saggitarius A black hole should be close to 4 million m/s^2, or about 1/75th of the speed of light in one second.